Spleen cells (SpC) from nonimmunized rabbits homozygous for one allotypic specificity at the a heavy chain locus and/or for one allotypic specificity at the unlinked b kappa light chain locus were incubated in vitro with RNA extracts of lymphoid tissues from immunized rabbits of different genotype and cultured in the presence or absence of SRBC antigen or BSA antigen, to yield IgM and IgG immunoglobulin. The kinetics of immunoglobulin of host and RNA donor's allotype synthesized and secreted were quantitated over a period of 4 hr to 7 days of culture, in supernatant fluids by radialradioimmunodiffussion methods; 2) by inhibition assay: one of precipitation of specific I125 Fab b4 or b5 light chain or a heavy chain determinants and two by inhibition of neutralization of b4 or b5 Fab coated T4 phages. At the cellular level the allotype in the plaque forming cells (PFC) was determined by reactions (radioautography with I125 Ig antibodies) or inhibition with specific antibodies. Both the host and the RNA donor's allotype were found in most of the IgM and IgG PFC, in rosette foming cells (RFC) and in cell lysates. However, the donor allotype was detected in ng quantities only in supernatant fluids and accumulated with time up to 5 days of culture. Studies in the presence of inhibitors of RNA, protein or DNA synthesis (actinomycin D, puromycin or cytosine arabinoside) have clearly indicated the necessity of de novo protein synthesis for the expression of the RNA donor's allotype. Although de novo RNA synthesis may not be necessary for the expression of allelic allotype on short cell structures, other data do not exclude this necessity on longer term cultures. RNA extracts or RNA treated cells were also injected into rabbits (in vivo) differing in genotype from the RNA donor. The RNA donor's allotype was found in 24-33% of the IgM plaques after 5 days (without subsequent SRBC injections) and in 25-42% of the IgG plaques after 37 days (following subsequent SRBC stimulation). The synthesis of allogeneic immunoglobulin by lymphoid cells after interaction with RNA in vivo or in vitro implies an informational molecule in the RNA extracts. Since the RNA extracts are inactivated by RNase but not by DNase or trypsin, this molecule appears to be an RNA moiety.